easiest way for me: draw a box, set it to the desired transparency, set it to the side out of artboard, then draw your box, set the stroke to desired color and point, then when you set your fill color, just use the color picker (eyedropper tool) and choose the color from your previous box. You will then have your box with desired opaque stroke and your fill will be set to desired transparency. You can then delete the box that was set off to the side.